Image sensing devices, such as a charge-coupled device (CCD), are commonly found in such products as digital cameras, scanners, and video cameras. These image sensing devices have a limited dynamic range when compared to traditional photographic film products. A typical electronic image sensing device has a dynamic range of about 7 stops. This means that the exposure for a typical scene must be determined with a fair amount of accuracy in order to avoid clipping the resultant signal. By contrast, natural scenes often exhibit a dynamic range of 9 stops and higher. This is mainly a consequence of multiple light sources with widely varying intensities illuminating the scene objects. Specular highlights also contribute to the dynamic range of natural scenes.
Electronic sensors used to scan photographic film must also contend with a high dynamic range of signal intensities. In U.S. Pat. No. 5,221,848 issued Jun. 22, 1993 to Milch entitled High Dynamic Range Film Digitizer and Method of Operating the Same discloses a method and apparatus designed to extend the dynamic range of an electronic image sensor. Intended primarily for scanning photographic film, the system described by Milch includes a one-pass film scanner using a charge-coupled device scanner having a plurality of linear arrays having the same spectral sensitivity. One of the arrays has a faster response to light than the other array. The information from the two arrays is then combined and digitized forming an extended dynamic range digital image.
Digital electronic cameras employ a single image sensor with a color filter array (CFA) to produce a sparsely sampled digital image. A typical color filter array pattern is disclosed in U.S. Pat. No. 3,971,065 issued Jul. 20, 1976 to Bayer entitled Color Imaging Array. Interpolation algorithms are employed to produce a full resolution color image from the sparsely sampled image. Digital cameras also need to record scenes having a high dynamic range. One way to obtain a high dynamic range image from a digital camera is to employ a high bit depth analog to digital converter in the camera. Another way is to employ an image sensor having interspersed fast and slow photosites as disclosed in copending U.S. Ser. No. 09/615,398 filed Jul. 13, 2000 by Gallagher et al. which is incorporated herein by reference.
It is known to employ a tone scale function that is specifically designed for use with extended dynamic range digital images. For example, U.S. Pat. No. 5,822,453 issued Oct. 13, 1998 to Lee et al. entitled Method for Estimating and Adjusting Digital Image Contrast discloses a method of calculating a tone scale function using the pixel values of an extended dynamic range digital image involving estimating the scene contrast from the digital image. However, the method taught by Lee et al. is not optimized for images that are captured by extended dynamic range image sensors having fast and slow photosites.
Therefore, there exists a need for an improved method of performing tone scale modifications on extended dynamic range images of the type that are captured by extended dynamic range image sensors having fast and slow photosites.